1. Field of the Invention
The present invention relates to a transmission control apparatus for controlling data transmission in a data transmission system for sending or receiving data through signal transmission channels.
2. Related Background Art
There are already proposed various data transmission systems for transmitting image data or computer generated data among plural stations through signal transmission channels.
The signal receiving station in such data transmission system identifies the reception of a signal even for a single pulse, for example, due to noise on the signal transmission channel, and it is identified as an error signal only after predetermined signal processing. In this manner the identification of an error signal requires a considerably long time.
Also a modulation process involving a time factor and a data factor such as Manchester encoding is often applied to the transmitted data, but distortion in the received signal will result in errors in the resulting time signal and data signal, eventually leading to incorrect data.
Also in a transmission control apparatus, a sending or transmitting circuit and a receiving circuit are respectively connected to signal transmission lines exclusively assigned for transmission and reception, so that the receiving circuit is not affected by the signal from the sending circuit.
However in a system utilizing a signal transmission channel common to signal sending and receiving, such as a signal bus, the sending circuit and the receiving circuit are connected to a common signal transmission channel, so that the signal emitted by the sending circuit is received also by the receiving circuit of the same station. Consequently the transmission control circuit has to control not only the sending circuit but also the receiving circuit against the signal emitted by itself, thus resulting in a loss in control efficiency.
Also the use of a long signal transmission channel often results in an error due to reflected signals, caused by a fact that an emitted signal is reflected at the end of the channel and is received, with a time delay, by the receiving circuit of the same transmission control circuit.
Besides, certain receiving circuits tend to cause distortions in signals by an abrupt change in the signal transmission channel from a signal-free state to a signal-containing state, thus generating error signals.
Also certain signal receiving circuits terminate the signal processing by detecting a signal pattern, generally called postamble, attached at the end of received signal for indicating the end of signal.
However, if such a signal pattern is not received by some reason, the signal processing circuit continues to function and may detect a part of the signal patterns present in the subsequently received signals as the end signal pattern, thus resulting in a loss in the subsequently received signals.
It is therefore conceivable to terminate the signal processing by detecting the end of signal reception from the signal transmission channel, but such method is still defective in that a short distortion in the received signal or a short absence of a received signal may be judged as the end of signal.